Decade Volcano: Unzen

Chances are that you only know Japan’s Unzen Volcano from a video that shows a huge pyroclastic flow surging over ridges as people in the foreground flee in terror.

And that did happen at Unzen in 1991. While the people we see in it survived, the fiery gray cloud of pulverized rock and searing gases had just killed 40-some others, including volcanologists Harry Glicken and Maurice and Katia Krafft.

Here is the bigger picture behind that video clip:

Glicken was a pioneer in volcanic avalanche studies. The Kraffts were there that day to obtain more footage for a public education film about volcano hazards. (Oregon State University; Thompson)

That killer cloud, while big and unusually mobile, was only one of thousands of pyroclastic flows during Unzen’s 1990-1995 eruption.

Why so many? It has to do with this Decade Volcano’s structure and how it erupted then.

Unzen is a scenic complex of hills — these are actually lava domes — and low stratovolcanoes (the pointy type, though none more than 5,000 feet high).

Besides being Japan’s first national park and a UNESCO Geopark, Unzen covers most of Kyushu Island’s Shimabara Peninsula, east of Nagasaki (yes, that Nagasaki).

Volcanic activity may have started on the southern part of the peninsula 4 million years ago, eventually reaching its present center around one of the Unzen stratovolcanoes (Mount Fugen, or Fugendake) some 500,000 years ago. (Kyushu University Internet Museum)

In 1990, Unzen awoke from a two-century sleep with earthquake swarms and then, in November, a steam-driven explosive eruption at Fugendake. Following this came dramatic water/magma blasts for about six months.

Then the volcano began to extrude from its summit lots of sticky gray lava, which accumulated atop Fugendake as a large dome.

This was very unstable, of course, and it frequently collapsed in pyroclastic flows. But the dome drew a lot of scientific attention because such lava more often erupts explosively.

This was one of the first eruptions monitored in-depth by volcanologists from start to finish. They accurately predicted the time and location of first lava emergence and got people out of harm’s way before burning ash clouds began to speed down the slopes, though nothing could be done to prevent the resulting $2 billion in property damage. (Nakada and Eichelberger)

The Krafft/Glicken group was inside an evacuation zone, but no one yet understands why that particular flow was more powerful than the rest.

Our lesson is clear: heed official warnings, no matter how fascinating the eruption may be.

Unzen is one of two Decade Volcanoes that infamously killed world-class experts during the International Decade that was declared, in part, to reduce volcanic risk (we will meet the other one – Galeras – next week).

The Decade Volcano program, though, did lead to

. . . a number of new insights about eruptive processes and possibilities for forecasting explosive eruptions. At Unzen, eruptions began shortly after the Decade, only a few kilometers from Shimabara City . . . the death toll was kept low by timely warnings and evacuations, though underestimation of risk by a few residents and others did lead to about 45 unfortunate deaths.

— Chris Newhall (in source list)


32.761° N, 130.299° E, in Nagasaki Prefecture, northwest Kyushu, Japan. The GVP Volcano Number is 282100.

Nearby Population:

Per the Global Volcanism Program website:

  • Within 5 km (3 miles): 6,979
  • Within 10 km (6 miles): 76,424
  • Within 30 km (19 miles): 444,737
  • Within 100 km (62 miles): 7,313,362

Current Status:

Aviation Code Green, normal.


  • Eruption styles: Explosions, lava flows, pyroclastic flows, debris flows, and lahars (ashy mud flows).
  • Biggest recorded event: In 1792, a large earthquake during an eruption at Fugendake triggered the partial collapse of Mayuyama lava dome. Debris cascaded into the sea, causing a 180-foot-high tsunami. Altogether about 15,000 people died in what was Japan’s worst volcanic disaster in recorded history.
  • Most recent eruption: 1996.
  • Past history: See the GVP for details. There are no Plinian events in the list. Volcanoes don’t have to have high-VEI numbers to be terrible. The lethal and costly 1990-1995 eruption was “only” VEI 1 and the most recent activity was VEI 2, as was the 1792 eruption in which so many lost their lives.


The Japan Meteorological Agency and the Shimabara Earthquake and Volcano Observatory (Japanese).

Featured image: NASA/JPL


Oregon State, Volcano World page on the Kraffts.

Oregon State, Volcano World. 2020. Unzen.

Unzen Tourist Association. n.d. tp:// Last acessed April 2, 2020.

Wikipedia. 2020. Mount Unzen. Last accessed April 2, 2020.

University of Tokyo Volcano Research Center and Kyushu University Shimabara Earthquake and Volcano Observatory. 1999. Welcome to Unzen Decade Volcano.

Kyushu University. 1998. Internet Museum: Eruption of Unzen Volcano and its Background.

Nakada, S., and Eichelberger, J. 2004. Looking into a volcano: Drilling Unzen.

Geological Survey of Japan, AIST. n.d. Active Volcanoes of Japan: Unzen Volcano. Last accessed bbn April 2, 2020.

Global Volcanism Program. Last accessed March 31, 2020.

Newhall, C. 1996. IAVCEI/International Council of Scientific Union’s Decade Volcano projects: Reducing volcanic disaster. status report. US Geological Survey, Washington, DC. Retrieved from

Oregon State University: Volcano World. 2020. Unzen. Last accessed April 2,2020.

Thompson, D. 2002. Volcano Cowboys: The Rocky Evolution of a Dangerous Science. St. Martin’s Griffin (this edition:

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.